This invention relates to oscillators such as local oscillators for use in radio receivers and particularly to those adapted for integrated circuit construction and designed to be used with tank circuits including varactor diodes.
It is well known in the art of radio design that varactor diodes, whose capacitance varies with applied voltage, are useful in tuned circuits such as local oscillators. However, there are also well known problems associated with such use. One of such problems is that of voltage variation across the varactor diode.
A varactor diode used in a tank circuit for a local oscillator in a radio receiver is generally supplied with a DC bias voltage thereacross to produce a predetermined capacitance. However, the alternating variations in voltage across the tank circuit associated with oscillation cause corresponding changes in this bias voltage across the varactor. The relationship of voltage to capacitance in a varactor is not linear; and therefore the effect of these oscillations can be to shift the average voltage across the varactor away from the predetermined bias. This can lead to mistracking between the local oscillator and the RF stages of the radio if the oscillations across the tank circuit are not maintained at a low level.
In addition, modern radio receiver manufacturing recognizes the advantages of integrated circuits in terms of size, weight and cost of the receiver. Although it is not generally practical to construct tank circuits in this manner, it is desirable to include the remainder of the oscillator components on an integrated circuit, which circuit may include most of the circuit elements of the receiver.